A thermal type flowmeter that measures the flow rate of a gas such as air is provided with a flow rate measurement unit for measuring a flow rate and is configured to measure the flow rate of a gas through heat transmission between the flow rate measurement unit and the gas, which is a measurement target. The flow rate measured by the thermal type flowmeter is widely used as important control parameters of various devices. A characteristic point of the thermal type flowmeter is that the thermal type flowmeter can measure the flow rate, for example, the mass flow rate of a gas at a relatively high accuracy in comparison with other types of flowmeters.
Meanwhile, there is a demand for further improving a gas flow rate measurement accuracy of the thermal type flowmeter. For example, for a vehicle in which an internal combustion engine is installed, there is a very high demand for fuel saving or exhaust gas purification. In order to meet the demand as described above, it is needed to measure the amount of inhaled air, which is a main parameter of the internal combustion engine, at a high accuracy.
A thermal type flowmeter that measures the amount of inhaled air that is guided into an internal combustion engine is provided with a sub path into which a portion of the amount of inhaled air is taken and a flow rate measurement unit disposed in the sub path, measures the flow rate of air flowing the sub path through heat transmission between the flow rate measurement unit and the air, and outputs an electric signal indicating the amount of inhaled air that is guided to the internal combustion engine. In addition, the sub path is configured such that a stream flowing into the sub path via an inlet branches toward a bypass outlet and a dust discharging port and dust flowing into the sub path via the inlet is discharged through the dust discharging port. Therefore, the flow rate measurement unit is prevented from being damaged by the dust colliding with the flow rate measurement unit.
For example, in JP-A-2012-202755 (PTL 1), as understood from [0023] and FIG. 1, a bypass flow path is provided with an intake inlet that is open toward an upstream side of an intake path in a direction in which an intake main stream flows, an intake outlet that is open toward a downstream side of the intake path in the direction in which the intake main stream flows, a straight path that linearly extends from the inlet and in which intake air proceeds straight in the same direction as the intake main stream in the intake path, and a circular path in which intake air having proceeded straight in the straight path circles and proceeds to the outlet. Note that, the straight path is linearly connected to a dust discharging path for discharging dust and a downstream end of the dust discharging path is formed with a dust discharging port that is open toward the downstream side of the intake path in the direction in which the intake main stream flows.
In addition, in JP-A-2013-190447 (PTL 2), as understood from [0020] to [0028] and FIGS. 1 and 2, an air flow rate measuring device is provided with a flow rate sensor disposed in a sub bypass flow path that branches off from an intermediate portion of a bypass flow path. Furthermore, a line perpendicular to an opening surface of a bypass outlet is provided to extend in a direction away from a junction. That is, the opening surface does not face a main stream downstream direction straight and the opening surface faces a direction that is inclined downward in a height direction with respect to the main stream downstream direction. Since a downstream end of a second wall surface is positioned on an upstream side of a downstream end of a third wall surface in a direction in which a bypass stream flows, a possibility that dust that is reflected after colliding with the third wall surface collides with the second wall surface is low and dust colliding with the third wall surface is less likely to be discharged via the bypass outlet. Accordingly, it is possible to prevent dust from reaching the flow rate sensor.